The Beast, is, as Golding's final page has it, "the darkness of man's heart". It's the evil, the fundamental badness, inside all human beings - which has to be fought to prevent human civilisation descending into savagery. And what you're seeing when the "beast" talks here, is Simon hallucinating - remember that Simon suffers from epilepsy (and possibly other things as well) and has some sort of gift of prophecy.

"Maybe... it's only us", Simon says right at the start of the novel. And the Beast gives the same verdict on the situation: "You knew, didn't you? I'm part of you?". It's Simon's mind in dialogue with itself.

It's also a clever pun. The flies swarming around the pig's head give the illusion of movement, making the head seem alive. The head, then, becomes the "Lord" of the flies - which is a translation of "Beelzebub" or the Devil. So what symbolically speaks as "the darkness of man's heart" is also, literally, the lord of the flies - the devil.